In recent years, liquid crystal displays (LCDs) have been employed not only in notebook-sized personal computers (PCs) and monitors but also in ordinary televisions (TVs). For light-diffusing plates (thickness: 1 to 3 mm) which are incorporated in televisions to serve as a backlight source for ensuring sufficient brightness, use of acrylic resin plates is currently a mainstream, particularly for such plates of 20 inches or thereabouts.
However, since acrylic resin exhibits low heat resistance and high moisture absorption, dimensional stability is unsatisfactory, resulting in problematic warpage of a light-diffusing plate for a large screen size.
Thus, recently, polycarbonate (hereinafter may be abbreviated as PC) resin, which exhibits more excellent heat resistance and moisture absorption resistance as compared with acrylic resin, has been more widely employed as a matrix resin for light-diffusing plates.
Hitherto, PC resin light-diffusing plates have been generally formed through extrusion.
The reason why PC resin is extruded is that, when PC resin—having low flowability—is injection-molded to form a plate having a size larger than 20 inches, the molded products suffer bad color tone due to shear heating, as well as poor precision in thickness, lack of uniformity in thickness, etc., and that these problems are avoided through employment of extrusion.
However, extrusion is not advantageous in terms of cost. For example, sheet raw material obtained through extrusion must be subjected to secondary processing such as cutting to predetermined dimensions, or waste pieces may be produced by mismatch between a size of light-diffusing plates of interest and a given sheet width.
Therefore, there have been employed, as injection molding material which can reduce shape-processing cost as compared with extrusion material for forming a light-diffusing plate, acrylic resin, MS resin, and cycloolefin resin, which exhibit high flowability. PC resin, which exhibits excellent heat resistance and moisture absorption resistance but exhibits low flowability, encounters difficulty in injection molding to form a light-diffusing plate having a thickness of about 2 mm and a size larger than 20 inches.
Meanwhile, when employed as an optical plastic material, PC resin exhibits disadvantageously large birefringence. There has already been known an optical resin material for solving this problem, which material is formed through polymerization between a linear aromatic polycarbonate and an unsaturated compound essentially including an aromatic vinyl compound (see patent Document 1).
However, patent Document 1 does not disclose production of a light-diffusing plate including addition of a light-diffusing agent to PC resin.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 62-138514